docs: def Rao_Kumar_2007(Re, eD):
      r'''Calculates Darcy friction factor using the method in Rao and Kumar
      (2007) [2]_ as shown in [1]_.
  
      .. math::
          \frac{1}{\sqrt{f_d}} = 2\log\left(\frac{(2\frac{\epsilon}{D})^{-1}}
          {\left(\frac{0.444 + 0.135Re}{Re}\right)\beta}\right)
  
      .. math::
          \beta = 1 - 0.55\exp(-0.33\ln\left[\frac{Re}{6.5}\right]^2)
  
      Parameters
      ----------
      Re : float
          Reynolds number, [-]
      eD : float
          Relative roughness, [-]
  
      Returns
      -------
      fd : float
          Darcy friction factor [-]
  
      Notes
      -----
      No range of validity specified for this equation.
      This equation is fit to original experimental friction factor data.
      Accordingly, this equation should not be used unless appropriate
      consideration is given.
  
      Examples
      --------
      >>> Rao_Kumar_2007(1E5, 1E-4)
      0.01197759334600925
  
      References
      ----------
      .. [1] Winning, H. and T. Coole. "Explicit Friction Factor Accuracy and
         Computational Efficiency for Turbulent Flow in Pipes." Flow, Turbulence
         and Combustion 90, no. 1 (January 1, 2013): 1-27.
         doi:10.1007/s10494-012-9419-7
      .. [2] Rao, A.R., Kumar, B.: Friction factor for turbulent pipe flow.
         Division of Mechanical Sciences, Civil Engineering Indian Institute of
         Science Bangalore, India ID Code 9587 (2007)
      '''
      beta = 1 - 0.55*exp(-0.33*(log(Re/6.5))**2)
      return (2*log10((2*eD)**-1/beta/((0.444+0.135*Re)/Re)))**-2